The present invention pertains to the field of power supply circuit technology; in particular, to a power supply circuit suitable as an internal power supply of a liquid crystal driver IC.
A power supply circuit as a conventional liquid crystal driver IC circuit is indicated by symbol 102 in FIG. 3.
Symbol VDD in FIG. 3 and FIGS. 1 and 2 to be described later indicates a positive voltage line connected to a positive voltage source of 3V or so, and symbol VEE indicates a negative voltage line connected to a negative voltage source of xe2x88x9215V or so. In addition, symbol VSS is the ground potential.
The power supply circuit 102 has output transistor 111, amplifier 112, voltage detection circuit 120, and reference voltage circuit 125.
The output transistor 111 is configured with a p-channel MOS transistor, its source terminal is connected to positive voltage line VDD, and its drain terminal is connected to external output terminal 128. Its gate terminal is connected to the output terminal of amplifier 112.
Load 127 and voltage detection circuit 120 are connected to external output terminal 128, whereby when output transistor 111 becomes conductive as amplifier 112 outputs a low signal, a current is supplied to load 127. At this time, the voltage of external output terminal 128 is divided by resistor elements 121 and 122 in detection circuit 120 and input as detection voltage VS to the inverted input terminal of amplifier 112.
Reference voltage circuit 125 is connected to the non-inverted input terminal of amplifier 112 in order to input reference voltage Vref output from reference voltage circuit 125.
When detection voltage VS is higher than reference voltage Vref, the output voltage of amplifier 112 increases, the current driver capability of output transistor 111 drops, and the current flow therein drops, so that the voltage of external output terminal 128 drops.
On the contrary, when detection voltage VS is lower than reference voltage Vref, the output voltage of amplifier 112 decreases, the current driver capability of output transistor 111 increases, and the current flow therein increases, so that the voltage of external output terminal 128 increases.
The voltage of external output terminal 128 is regulated to a fixed voltage by the negative feedback operation of amplifier 112 in said manner.
In terms of the configuration of reference voltage circuit 125, reference voltage circuit 125 has bias circuit 140 and voltage generator circuit 130.
The bias circuit 140 is configured by connecting diode-connected p-channel type MOS transistor 141 in series with resistor element 142, and the differential voltage between positive voltage line VDD and ground voltage VSS is applied to bias circuit 140. Here, a voltage obtained by subtracting the operating voltage of MOS transistor 141 from the differential voltage is applied to resistor element 142.
Because the voltage of positive voltage line VDD is almost fixed, a constant voltage is applied to resistor element 142, and a constant current of a fixed amount flows therein. Said constant current also flows into MOS transistor 141.
Reference voltage generator circuit 130 has p-channel type MOS transistor 131 and n-channel type MOS transistor 134.
P-channel type MOS transistor 131 includes a current mirror circuit together with diode-connected MOS transistor 141 in bias circuit 140, and a current proportional to the current flowing in diode-connected MOS transistor 141 flows into said MOS transistor 131.
In addition, n-channel type MOS transistor 134 is diode-connected, and a current flows therein from MOS transistor 131 constituting the current mirror circuit. As a result, a constant voltage close to the threshold voltage is generated at both ends of diode-connected n-channel type MOS transistor 134.
The constant voltage serves as reference voltage Vref and is input to the inverted input terminal of amplifier 112.
In the case of power supply circuit 102, its current supplying capability needs to be determined according to the maximum current consumption by load 127. Because said load 127 is an internal logic circuit provided in a liquid crystal driver IC, and the size of output transistor 111 needs to be increased, the power consumption of power supply circuit 102 is difficult to decrease.
In addition, because amplifier 112 requires an internal capacitor for phase compensation, a large area is needed, resulting in high cost.
The present invention was created to solve the problems of the prior art, and its objective is to present a low power consumption, reduced size power supply circuit.
In order to solve the problems, the power supply circuit of the present invention is provided with a current supply circuit having a first current supplying element connected in parallel to a current supply terminal and a second current supplying element configured with a transistor, a detection circuit containing a clamping circuit provided with multiple diode circuits and connected in series with the current supply circuit in order to output a detection signal according to the voltage at the current supply terminal, and a control circuit having the second current supplying element, a first transistor including a current mirror, a second transistor which becomes conductive or non-conductive according to the detection signal, and a third transistor connected in series with the first transistor in order to control the conduction status of the first transistor according to the conduction status of the second transistor.
Also, in the power supply circuit of the present invention, it is desirable that a bias circuit which outputs a bias voltage be provided, and that the bias voltage be applied to the control terminal of the transistor of first current supplying element.
Moreover, it is desirable that the bias circuit have a fourth transistor which includes a current mirror together with the first current supply circuit, and that the control circuit have a fifth transistor which constitutes a current mirror circuit together with the fourth transistor while supplying supply current to the second transistor.
The power supply circuit of the present invention is included in the diode circuits that are by diode-connecting MOS transistors, and the diode circuits are connected in series to form the clamping circuit; and the diode-connected MOS transistors form a current mirror together with the second transistor (detector transistor).
The detector transistor enters either a conductive or a shut-off status according the status, that is, either conductive or shut-off, of the clamping circuit, whereby the conduction status of the first transistor (driver transistor) is controlled in order to control the operation of the second current driver element.
In addition, the clamping circuit has a function to maintain the voltage value at the current supply terminal at an almost fixed voltage.